Mysql

Indexes are supposed to make queries fast. Sometimes they make them slower. Here’s what I wish someone told me before I tanked production performance trying to “optimize” our database. The query that started it all Support said the admin dashboard was timing out. Found this in the slow query log: SELECT * FROM orders WHERE customer_id = 12345 AND status IN ('pending', 'processing') AND created_at > '2025-12-01' ORDER BY created_at DESC LIMIT 20; Took 8 seconds on a table with 4 million rows. Obviously needs an index, right? ...

Sizing Pool size ≈ CPU cores * 2–4 per service instance; avoid per-request opens. For PgBouncer tx-mode: disable session features; avoid session-prepared statements. Timeouts & limits Set ConnMaxLifetime, ConnMaxIdleTime, MaxOpenConns, MaxIdleConns. Add statement timeouts; enforce context deadlines on queries. Instrumentation Track pool acquire latency, in-use/idle, wait count, timeouts. Log slow queries; sample EXPLAIN ANALYZE in staging for heavy ones. Hygiene Use prepared statements judiciously; reuse sqlx.Named/pgx prepared for hot paths. Prefer keyset pagination; cap result sizes; parameterize everything. Checklist Pool sized and monitored. Query timeouts set; slow logs reviewed. No per-request connections; connections closed via context cancellation.

Query hygiene Add composite indexes matching filters/order; avoid leading wildcards. Use EXPLAIN to verify index usage; watch for filesort/temp tables. Prefer keyset pagination over OFFSET for large tables. Config basics Set innodb_buffer_pool_size (50-70% RAM), innodb_log_file_size, innodb_flush_log_at_trx_commit=1 (durable) or 2 (faster). max_connections aligned with app pool size; avoid connection storms. Enable slow query log with sane threshold; sample for tuning. App considerations Reuse connections (pooling); avoid long transactions. Limit selected columns; cap payload sizes; avoid large unbounded IN lists. For read-heavy workloads, add replicas; route reads carefully. Checklist Indexes audited; EXPLAIN reviewed. Buffer pool sized; slow log enabled. Pagination and payloads bounded; connections pooled.